RELATIONS AMONG SPECIES 



379 



the southern migration was not continuous over the 

 United States. 



Owing to these changed conditions, the Great 

 Plains and lowlands of the eastern Rockies developed 

 a unique climate and a unique grassland and shrub 

 flora, the Subtropical Scrub Element. This element 

 acted like an indented finger that separated the 

 Arcto-Tertiary Geoflora distribution between middle 

 Colorado and eastern Texas as far north as southern 

 Canada. As a result of the scrub barrier, only west- 

 ern North America contained species whose closest 

 living relatives now are found in eastern Asia. The 

 barrier apparently prevented any of these species' get- 

 ting into the eastern United States or Mexico. The 

 effectiveness of the southern part of the barrier is 

 indicated by the fact that prior to the barrier western 

 North American species did enter Mexico. For ex- 

 ample, the early Eocene floras of the Pacific States 

 have their closest living relatives in the highlands of 

 Mexico and Central America. 



The majority of mammals, owing to late Eocene or 

 early Oligocene events, were of modern types. The 

 condylarths and amblypods were gone, the last creo- 

 donts and archaic primates soon disappeared, and 

 marsupials and insectivores assumed their present 

 subordinate role. Unfamiliar forms were mostly 

 among the hoofed mammals. More advanced horses, 

 rhinoceroses, oreodons, and huge titanotheres were 

 dominant. However, bats maintained the modern 

 form they gained in the Eocene, and modern carni- 

 vores and rodents truly dominated the scene. 



Miocene. The Miocene marks the start of rapid 

 growth of the western mountains. This mountain 

 building, which became most pronounced in the late 

 Pliocene, led to climates, and thus to floras and 

 faunas, which progressively assumed a more complex 

 pattern of distribution and more fixed zonation (Fig- 

 ure 19.16). The pattern and zonation intensified bar- 

 riers and caused rapid differentiation and evolution 

 of life. In addition, the entire period to latest Plio- 

 cene continued the general trend of cooling and 

 drying. 



Findings of geologists imply the following: The 

 Rockies upwarped into a low fold hundreds of miles 

 broad, a trend culminating in the Pliocene. Although 

 the Sierras were the site of a broad plateau only 3000 

 feet above sea level (western Nevada was at about 

 2000 feet), folding and faulting prevailed in the Coast 

 Ranges, adjacent ranges, and Puget .Sound Basin. 

 Elsewhere, faulting was the major force, generally 



Figure 19.16 Miocene paleogeography, including the distribution of 

 geofloros. Symbols as in Figure 19.14 plus grassland in present Great 

 Plains area. 



contributing to elevation and vulcanism. Of great 

 significance was the extensive volcanic activity of this 

 time. Areas near the Pacific Ocean from Alaska 

 down into California were extremely active. The 

 Columbian Plateau of eastern Washington, eastern 

 Oregon, and southern Idaho gained much of its pres- 

 ent form except as it has been modified by erosion; so 

 did the Cascade Range. Further activity took place 

 in the Basin and Range province just to the south of 

 the Columbia Plateau and in the Colorado Plateau 

 province just to the southeast of the Basin and 

 Ranges. This activity extended even into Mexico. In 

 fact, most to all of the areas discussed had significant 

 vulcanism well into the Pleistocene. 



Although this period featured a mixed deciduous- 

 coniferous forest in the northwestern states and Great 

 Basin, a flora with modern remnants restricted to 

 China and the Appalachians, the Middle Miocene 

 was the start of grassland development. However, a 

 full-fledged Grassland Element did not exist until the 



